Mar. 25, 1514 Tyioses in American Woods 447 



the wood produced by felling the tree may have an important bearing on 

 the presence of tyloses in the outer rings of a log, where the parenchyma 

 cells are still living and capable of growth. It is possible to find in these 

 rings young or old, or large and small, tyloses together in the same vessel. 

 (PI. LIV, R$.) Although exceptions have been noted, the idea that a 

 considerable number of the outer rings are entirely free from tyloses 

 has, however, been very generally accepted (Strasburger). 1 The data 

 obtained from the present study show that there is a very considerable 

 formation of tyloses in the outer rings of the sapwood. The question 

 then arose as to whether these sapwood tyloses were of normal origin or 

 whether they were due to some wound stimulus, such as the felling of 

 the tree. It was finally concluded that they were normally formed 

 tyloses, because their development throughout the vessels was very 

 uniform instead of being sporadic or irregular, as in the case of tyloses 

 associated with wounds (PL UV, Ri and #2), and because an exami- 

 nation of branches from living trees of Rhus, the sumach, Catalpa, and 

 Robinia, the black locust, made immediately after cutting, confirmed the 

 other observations of the relatively early formation of tyloses in many 

 species. In material which was not received for examination until 

 several weeks after it was cut, thin, irregularly distributed tyloses were 

 often found in the outer vessels, though the latter must have been func- 

 tioning in sap condition at the time the tree was felled. 



It is noteworthy also that in this study tyloses were found to reach the 

 most remarkable development in ring-porous woods, such as oak, hickory, 

 black locust, or osage orange. (PI. LIU, figs, i and 3, and PI. LVI, 

 fig. 2.) In woods where tyloses are few and scattered there is consider- 

 able variation from specimen to specimen in the actual number of tyloses 

 present. This tendency is clearly shown in the woods of the diffuse 

 porous group. (Table II.) It is also noticeable that in the two or three 

 rings surrounding the pith in a diffuse porous wood tyloses are often 

 much more abundant than elsewhere in either the heartwood or sapwood. 



EFFECT OF THE DISTRIBUTION OF PARENCHYMA TISSUE 



Since tylose formation depends upon the presence of parenchyma cells 

 either in the form of wood parenchyma or medullary rays in close prox- 

 imity to vessels or tracheids, the variation in position, abundance, and 

 vitality of these cells affords at least a partial explanation of the irregular 

 development of tyloses in different species of wood. Parenchyma tissue 

 is considerably developed in the following families and their respective 

 genera. 2 This study has shown that in these families are a large number 

 of native woods exhibiting tyloses. 



1 Tyloses are ... instrumental in closing the water courses ot the heartwood. . . . These are intrusive 

 growths from living cells which penetrate the cavities of the adjoining tracheal elements during the transi- 

 tion of sapwood into heartwood. 



2 Solereder, Hans. Systematic Anatomy of the Dicotyledons ... v. 2, p. 1143- Oxford. 1908. Certain 

 other woods with abundant parenchyma frequently produce gummy substances rather than tyloses. 



